A/N: Spoilers that pertain to the movie are purely visual, nothing plot-based.
Headcanon specific for this story: Thranduil has recently (whatever that might mean for a thousands-of-year-old elf) received his scar, so he still has to actively work for this glamour to conceal it because it doesn't take as much for it to fall.
"Do not talk to me of dragon fire. I know its wrath and ruin. I have faced the great serpents of the north."
Night had fallen over the forest. A thick cover of branches let it only partial chinks of silver moon and starlight, spotting the halls here and there with their brightness. In the king's halls, the usual merriment was being enjoyed, wine and food being passed among the elves.
Three were absent from the nightly celebration, though. The royal family, no less. Thranduil sat on his throne with Legolas sitting on him, tinkering with a small bow and arrow. He hefted it up and assumed his best archer stance.
"How's this, Ada?" Legolas asked.
Thranduil laughed softly. "You won't hit anything sitting down, ion. You must stand strong, as a brave warrior."
Legolas slid off his father's legs and turned around to look at him. "Like you?"
Thranduil smiled in response. Legolas beamed and faced the front again with his second attempt. "I'll be brave as you someday, Ada," he declared stoutly. "Maybe someday even more."
"I have no doubt you will. Now—that's better." He nodded approvingly as his son's posture, who jumped immediately out of position in excitement.
"Tell me, Ada, do you think one day if I can be as brave as you, I'll go on fantastic adventures as you have?"
Thranduil gave a light scoff. "Since when I have gone on fantastic adventures, ion? I've done nothing of the sort."
"The guards told me," Legolas said. "That is to say, I overheard them talking about them." He dropped his little bow and reclaimed his seat on Thranduil's lap.
"Were you sneaking around in the halls again? You are a clever if not mischievous elfling," Thranduil teased. "I'll need to speak to your mother about keeping a better eye on you. Very well, I concede that I have seen and done many things in my life, though I would not seem any of them fantastic—"
"Fighting a dragon sounds fantastic to me! And the serpents."
Thranduil froze. "More tales you've been hearing from the guards?" he said as lightly as possible.
"Maybe..." Legolas said, lying down with his head hanging over his father's legs. "Tell me, Ada, please?"
"Goheno nin," Thranduil apologized gently. "It is a story too sensitive for you."
Legolas's face fell. "I want it!"
"No." Thranduil spoke more firmly.
"No. It is full of violence and terror and pain—" Thranduil stopped suddenly. The left side of his face was burning, a searing agony. Just the mention of it had been enough for the magic to weaken. No, he thought. Not in front of Legolas—
The pain was multiplying by the second. Thranduil grimaced. He suddenly brought his hand up to his face, felt the burn, its blistering pain—there was fire everywhere—
"...Ada?" Legolas said slowly. He was waiting for Thranduil to continue. Though young, his keen ears had picked up the sound of the movement.
"No..." Thranduil said through gritted teeth. "Don't look-" The pain was threatening to overwhelm him but he had to conquer it before Legolas saw. It would surely scare him...Thranduil couldn't help but let a sharp exclamation of pain escape him.
The prince was slightly frightened then. "What are you hiding from me, Ada…?" He still had not turned, obediently obeying his father's wishes. But curiosity was itching at him. Legolas made his hands into fists.
The sight that met him was the most gruesome he had seen. His father's face was almost half gone, revealing only a cavern of muscle and bone. It spread the entire length, infesting the side of his lip and up to his temple. His left eye was milky white, void of any warmth or affection. It was as if half of his face had simply been corrupted and corroded away, into an ugly waste. What appeared to still be whole of him was contorted in pain.
Legolas stared at it for a moment, fear settling into his own face. Thranduil raised his gaze a little, looking at Legolas through the pain. "Legolas—" But the realization that this horrible thing had just said his name filled Legolas with terror. He seemed unable to recognize who it was, only that it had been his father just minutes ago. When Thranduil attempted to reach forward, he screamed.
Legolas backed away, forgetting he was sitting on his father, falling onto the ground. But he ignored the pain, scrambling to his feet. Thranduil remained where he was, leaning over, attempting to regain control.
Some guards had heard Legolas. Three ran into the hall, bows drawn. His mother was behind him. "Nana!" Legolas cried, running towards her.
"Shh, my child, what is it?" she soothed.
"Something's wrong, his face—!" Legolas pointed but was hardly able to keep his hand steady.
Meluihel looked up sharply and saw her husband bent over with pain. She got the attention of one of the guards. "Stay with him. Don't leave," she ordered, indicating Legolas. The guard complied, if not awkwardly, as Legolas immediately took hold of the hem of his tunic.
She approached Thranduil and knelt down next to him. "Hervenn?"
"I'm all right," he rasped. It was true: the magic was returning. She had a calming presence with her; Thranduil has felt it upon her arrival. "I'm all right," he repeated. The last of the magic concealed the wound. "But Meluihel, Legolas—"
She smiled and stood. "Thank you," she said to the guards. "All is well here. You are dismissed. Yes, you as well," she added, addressing the hesitant guard standing next to Legolas.
"Come up, ion," she said. "Av-'osto. You are safe. It has passed."
Legolas was still shaken. He approached the throne slowly, his wide eyes never leaving his father's. He meant to go to him, but Legolas couldn't forget the deep gouge in his father's face, the sightless white eye, the look of pure pain, and he instead clung to his mother's skirts.
"Av-'osto," Meluihel said again, softly. She took Legolas's hand and drew him closer to Thranduil. More secure with his mother by, Legolas climbed to where he had before, though not without some trepidation. He crouched on his knees and cautiously surveyed his father. Thranduil was alarmed by the intensity in Legolas's eyes. They were full of anxiety and shock. He felt them roving over his face until they settled on where the burn wound was.
Legolas reached out slowly, further and further, until his hand touched Thranduil's skin. The king was relieved to feel that his son's touch was warm still. There was something about the child's purity that put him at ease, giving him a stronger hold on the magic; now he resolved to put Legolas at ease.
Legolas felt every inch of Thranduil's cheek. The skin was soft again, his eye restored to color and life. Legolas tentatively looked into the pair of them, clear enough to see his own reflection.
"Does it hurt?" he murmured.
Thranduil was surprised, but reassured. "No," he answered quietly. "Not anymore."
Legolas removed his hand, satiated. "Good. I don't like to see you like that."
"You won't ever again," Thranduil swore, meaning it. He would never repeat putting his son through that distress.
"I think that's enough story time for today," Meluihel said, and gathered Legolas into her arms. "You need to rest."
"Meluihel, wait." She turned to see Thranduil had stood. He approached Legolas and smoothed his hair. "Ion, you must know that whatever I look like, you need not ever be afraid of me. I'll die before I do anything to hurt you."
Legolas nodded, comforted, but he looked ashamed then. "I know. I'm sorry, Ada. I said I'll be brave and I wasn't."
"No…it's not your fault. Save your bravery for another day," Thranduil said. "Be a child yet. Don't jump to do anything before you've spent your joy on another. Even an elf cannot be everything in the world. Play with your friends, sing with them to the birds. You'll be a warrior yet, and then you won't have time to do such things. Now is your chance."
Ada / daddy
Ion / son
Goheno nin /I'm sorry
Av-'osto / Don't be afraid